ARM: at91: sama5d3_xplained: spl: enable PMECC header generation

As sama5d3 xplained support the PMECC. So add the PMECC header for spl
binary. That make ROM loader can use PMECC to avoid error flips in spl
code in nandflash.

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Acked-by: Bo Shen <voice.shen@atmel.com>